This invention relates to a method for coating by glow discharge, and more particularly to a method for coating at least selected portions of the surface of an electroconductive workpiece with a reaction product containing a metal species or a semi-metal species by the glow discharge generated between the electroconductive workpiece connected to a cathode, at least one secondary cathode provided at a position near enough to the workpiece to generate interactions of glow discharge between at least selected portions of the workpiece and the at least one secondary cathode, and an anode.
The conventional method for coating an electroconductive workpiece includes a chemical vapor deposition (CVD) process and a physical vapor deposition (PVD) process and has been so far applied to coating with TiC and TiN. For example, according to the CVD process, for example, a workpiece, as heated to about 1,000.degree. C. in an electric furnace or by high frequency heating, is coated with TiC formed by catalytic reaction of the workpiece surface with a gas phase, for example, TiCl.sub.4 and C.sub.3 H.sub.8 in a gaseous state. The process of depositing the reaction product is a process of solid crystal growth on a workpiece from a gas phase, where there are differences in temperatures and concentrations of chemical species between the workpiece and the gas phase, and the degree of supersaturation is a driving force for a film growth. Thus, in the CVD process, the diffusion of gaseous reactants into a reaction boundary is based on a gaseous diffusion, and thus generally the concentrations of gaseous reactants at the boundary are low. In coating with TiC according to the CVD process, a film as thick as about 5-10 .mu.m is formed by coating at about 1,000.degree. C. for 2 to 3 hours, and thus a film-forming speed is low in spite of the high temperature. In order to form a film with stable characteristics, it is necessary to make fine control of mixing ratio, flow rate, etc. of gaseous reactants, and thus the coating process is complicated. Furthermore, such a high temperature as about 1,000.degree. C. is required, so that, when the workpiece is a most popular steel part, such deteriorations as embrittlement due to growth of coarse crystal grains, etc. are brought about. Their prevention needs complicated steps. Similar disadvantages also appear in the PVD process. In the PVD process, coating can be made at a low temperature, but decrease in adhesiveness of a film to a workpiece, decrease in film-forming speed, etc. appear as disadvantages in that case.
Coating of TiC by glow discharge has been disclosed [for example, F. J. Hazlewood and P. C. Iordanis. "Abrasion-resistant, titanium-carbide-based coatings formed by glow-discharge-assisted vapor deposition", paper 12, pages 29-37, Advances in Surface Coating Technology, International Conference of the Welding Institute and the Institute of Mechanical Engineers, London, Feb. 13-15 ('78)].
The disclosed coating method provides coating by glow discharge using an ionized gas, where TiCl.sub.4 is used as a gas source, and a film of TiC is formed from TiCl.sub.4 and C.sub.2 H.sub.2 as gaseous reactants and Ar+5% H.sub.2 as a carrier gas in a container under a reduced pressure of at least 10.sup.-1 Torr. Heating of workpiece is carried out by glow discharge energy, requiring no outside heat source. That is, the glow-generating surface itself is a heating source, and thus the temperature of a workpiece depends upon a proportion of surface area to volume of the workpiece. In other words, workpieces of identical, rather simple configurations can take a substantially uniform temperature distribution throughout the workpieces, and can undergo uniform coating, but workpieces having complicated configurations, particularly different proportions of surface area to volume, though the configurations are identical with one another, have local differences in ion collision energy and ionization density, making a temperature difference larger, and thus the concentration and depth of diffused atoms are widely fluctuated, giving a change to the film-forming speed. That is, uniform coating cannot be obtained. Particularly in workpieces with irregularity in configuration, glow discharge is concentrated at protruded parts that are liable to emit electrons, and thus the protruded parts are selectively coated with substantial failure to coat the recess parts.
These phenomena greatly depend upon discharge voltage at the glow discharge. That is, heating to higher than 600.degree. C. by glow discharge will rapidly increase the discharge voltage. The higher the discharge voltage, the more limited the direction of discharging electrons. That is, the glow discharge will be concentrated at positions that are more liable to emit electrons.
Furthermore, the difference in temperature is increased with increasing temperature, but the difference is not so remarkable at the now commercially applicable ion nitriding temperature of 600.degree. C., and thus there is no problem in the nitriding at that temperature. However, even in the nitriding at a high temperature, the difference in temperature becomes larger, and it is difficult to uniformly treat the desired portions of workpieces. To solve these problems, it has been proposed, for example, to carry out ion nitriding in the conventional vacuum heat-treating furnace or to conduct ion nitriding while using an outside high frequency heating. However, in the former case, heating of a workpiece is carried out by a heater such as carbon fiber heater, requiring a heat-treating power source of higher output with consequent reduction in heating by ions. That is, the ion collision energy to a workpiece will be less than in the conventional treatment only by ions, and the proportion of ion distribution to the surface of a workpiece is reduced. Consequently, the structure and control of a treating apparatus will be complicated, and total consumption energy will be increased, with a resulting reduction in concentration in atoms that take part in cleaning action by ions, film formation or hardening on the surface, etc.
In the latter case, when many workpieces are placed in a furnace for heating by induced current by high frequency, the individual workpieces have different heating temperatures, depending upon distances from the high frequency coil, and also the power source and control will be complicated as in the former case. Furthermore, the energy for the treatment is increased and the cleaning action by ions and control of ions on the surfaces of workpieces cannot be attained fully.
On the other hand, the entire workpiece is not always subjected to surface treatment of single function, but may be subjected to surface treatment of a plurality of functions within one and same workpiece, depending upon the intended application of the workpiece. In such treatment, the said ion surface treatment has not been carried out continuously in one and same furnace in one step, but has been carried out in a complicated process.
As a method of obtaining locally differently treated layers on a workpiece by ion-treating (for example, in different depths and hardness), an ion surface-treatment process is disclosed in the Japanese patent application Laid-Open No. 6956-1972 wherein an additional metal electrode (which forms an anode with respect to the workpiece) is inserted between the workpiece (cathode) and the wall of the vacuum container (anode) and is connected through a potentiometer to the positive terminal of the dc power supply so that changing the potential of the metal electrode by means of the potentiometer will partially vary the ion collision energy. With the process of e.g. ion nitriding, the additional metal electrode is provided in the vicinity of the desired portion of a workpiece which is to have a different nitriding layer, so that a change in potential of the metal electrode by means of the external circuit will provide a change in the ion collision energy at the desired portion to control the amount of nitrogen atoms that tend to diffuse into the portion, thereby forming a partially different nitrided layer. Since the nitrogen diffusion depends greatly on temperature, not on the ion collision energy in the case of such a method of changing the ion collision energy, it is greatly difficult to change the depth of the nitrided layer partially.
To solve these problems, the present inventors developed a process for carrying out the necessary surface treatment under a lower discharge voltage for a short period of time by providing a secondary cathode at a position near enough to an electroconductive workpiece to generate interactions of glow discharge between selected portions of the electroconductive workpiece and the secondary cathode, and generating glow discharge so as to cause the interactions between selected portions of the workpiece and the secondary cathode, thereby making the glow discharge voltage at the surface of the workpiece lower [U.S. patent application Ser. No. 174,748 filed Aug. 4, 1980, now U.S. Pat. No. 4,394,234, German Laid-open Application (DOS) No. 3,029,339]. That is, the prior art process provides a surface treatment process wherein glow discharge is established between a cathode and an anode of a power source to carry out heat treatment of a workpiece under a reduced pressure condition in a container, comprising the steps of placing the workpiece which has a conductive surface and which is electrically connected to the cathode in said container, positioning a secondary electrode which has a conductive surface and which is electrically connected to the cathode close to a selected treatment portion of said workpiece, and effecting a glow discharge between the conductive surfaces of said workpiece and the secondary electrode and said anode; the distance between the workpiece and the secondary electrode being selected to increase the surface temperature of the selected treatment portion of said workpiece and to increase the heat treatment effect on said selected treatment portion of said workpiece.
The present inventors have made extensive studies of applying a CVD process to the said surface treatment process as a series research and have established the present invention.